Transport across the plasma membrane (passive processes)
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What are the two types of PM transport processes? What is their difference?
- Active and passive
- Whether they require cellular energy or not
What is a passive process?
- A substance that moves down its concentration or electrical gradient due solely to its OWN kinetic energy
- No input of energy from the cell
What is an active process?
- A substance that moves 'uphill' against its concentration or electrical gradient which requires energy FROM the cell
- The energy provided by the cell is usually ATP
What are vesicles?
- Tiny spherical membrane sacks
- Used in endocytosis (join with PM) and exocytosis (break away from PM)
What are endocytosis and exocytosis?
- The process of a substance leaving or entering a cell by means of using vesicles (tiny membrane sacks)
- Endocytosis is a vesicle forming and detaching from the PM enclosing materials
- Exocytosis is a vesicle joining with a PM and depositing its contents in the ECF
What is diffusion? What allows diffusion in a cell?
- A passive process where a substance moves down its concentration gradient due to its kinetic energy (random movements).
- In a solution both the solute and solvent undergo diffusion.
- The PM has to be permeable to a substance for it to diffuse across it.
What factors influence the diffusion rate of substances across a plasma membrane (PM)?
- Temperature (All body's diffusion processes occur faster with a fever)
- Steepness of the concentration gradient
- Surface arena (of PM, larger+more surface area=faster)
- Mass of the diffusing substance (smaller molecules move more rapidly)
- Diffusion distance (greater the distance=slower)
What are the three types of diffusion?
- Simple diffusion
- Facilitated diffusion
What is simple diffusion? Examples? Edge cases?
- Passive process (yes all diffusion) in which uncharged nonpolar (hydrophobic) substances move freely through the lipid bilayer of the PM, without help of membrane proteins
- EX: O, CO2, steroids, nitrogen gasses, fatty acids, and fat-soluble vitamins (A,E,D,K)
- Some small polar molecules (H2O, urea, and small alcohols) pass through the bilayer by simple diffusion.
What is facilitated diffusion?
Passive process in which solutes move across the PM using integral proteins due to being too polar or highly charged to cross the lipid bilayer.
What is channel-mediated facilitated diffusion? What are common channels in typical PMs?
- Passive process in which an ion moves down its concentration gradient through a membrane channel (most are ion channels)
- In typical PMs the most numerous ion channels select for (K^+) and (Cl^-), with fewer available for (Na^+) and (Ca^2+)
What does it mean when a channel is gated?
- When a channel can change its shape to plug (or gate) its pore, or open it.
- Can happen randomly, or due to chemical or electrical changes in or outside of cells.
What is carrier-mediated facilitated diffusion?
Passive process in which an ion moves down its concentration gradient through a membrane carrier (transporter)
What is a transport maximum?
- An upper limit set on a cell that limits the rate of diffusion through carriers (transporters) due to the number of carriers.
- Once the transport maximum is reached the steepness of the concentration gradient does not increase the rate of diffusion (like a sponge completely saturated with water).
What is osmosis?
A solvent (water) diffusing through a semi-permeable (water permeable, not solute) membrane (plasma membrane)
What two ways does water pass through the plasma membrane during osmosis?
Water molecules can pass through cracks in the phospholipids or move through aquaporins (integral membrane proteins that function as water channels)
What is hydrostatic pressure? When is homeostasis achieved involving hydrostatic pressure?
- Pressure exerted against a semi-permeable membrane during osmosis on the side that is taking water in, which forces some water out.
- Homeostasis is achieved when the water leaving due to hydrostatic pressure and entering from osmosis are the same amounts.
What is osmotic pressure? What is it proportional to?
- The amount of pressure required to stop a solute (water) from permeating through a semi-permeable membrane.
- The pressure required to stop osmosisOsmotic pressure of a solution is proportional to the amount of solute that cannot pass the membrane (Higher solute concentration = higher osmotic pressure)
What is tonicity?
- A measure of a solutions ability to change the volume of cells by altering their water content (different osmotic pressure, solute%)
What is an isotonic solution?
Solution in which a cell maintains its shape and volume due to the same concentration of impermeable solutes on either side of the membrane.
What is a hypotonic solution?
- hypo=less than, tonic=tension
- Solution in which cells take in water, swell, then burst (hemolysis in RBC, lysis in other cells) due to a lesser concentration of solute than in the cytosol.
What is a hypertonic solution?
- hyper=greater, tonic=tension
- Solution in which cells loose water and shrink (crenation) due to a higher concentration of solute in the solution then in their cytosol.
What is hemolysis? Lysis? Crenation?
- Hemolysis- RBCs rupturing in hypotonic solutions
- Lysis- Cells other then RBCs rupturing in hypotonic solutions
- Crenation- Cells shrinking in hypertonic solutions
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